The present invention relates to processing methods in semiconductor device fabrication, and more particularly to methods for forming openings in a substrate, such as contact openings.
At several stages during wafer fabrication, it is necessary to form contact openings through insulative material to establish electrical communication with the integrated circuitry. To form contact openings, a masking layer such as a photoresist is typically formed over an insulative layer such as silicon oxide layer, and the contact opening is etched through the insulative layer by exposure to a plasma formed in a plasma reactor. A fluorocarbon plasma is typically used to etch silicon dioxide.
When dry etching a contact opening, the etchants also attack the resist layer causing the resist to gradually erode away, often before the desired depth of the opening is achieved. The erosion of the resist overlying the corners at the mouth of the opening can cause faceting or chamfering of the insulative layer from the contact with the etch gas, resulting in surface roughness and striations in the etch features, and the loss of critical dimensions of the opening being etched. In an array such as a memory cell, contacts are positioned in close proximity to each other, and the erosion and localized breakdown of the photoresist in an etch process can result in the development of notches and other blemishes in the surface of the contact, which can extend to and short circuit an adjacent contact or feature.
The stringent requirements of the etch process (such as rate, profile control, and selectivity to other films in the structure) may not permit that resist selectivity be made high enough to meet the conflicting manufacturing requirements that a) the resist be thin enough to be easily patterned with the lithography equipment in use, and b) the resist be thick enough to provide adequate masking for the etch process.
Therefore, a need exists for a method of etching silicon oxide layers to provide high aspect ratio openings that overcomes these problems.
The present invention provides processing methods for forming via or other contact openings using a self-aligned etch mask of a polymeric material.
The invention utilizes a self-aligning method to deposit a masking layer for a continued etch of a formed contact opening downwardly in a substrate, thus eliminating the need to something in production to compensate for a process with relatively high resist erosion. Typically, a higher selectivity process can be used. Such a process will be more expensive to operate because it will have a smaller process window and be more sensitive to variations in incoming material. Thus, it will generate more scrap and be more costly to operate. Alternatively, a hard mask under the resist layer may be used. Such a mask might be polysilicon or silicon nitride. These masks are costly to deposit, must be separately etched and subsequently removed.
The present method applies a polymerizing gas surface-wide over a substrate having a formed opening, and utilizes the height difference, or aspect ratio, between the base of the opening and the surface of the substrate to selectively deposit a polymer mask layer onto the surface of the substrate and the sidewalls of the opening to a limited depth. Thus, the polymer layer self-aligns relative to the pre-etched contact opening and serves as an etch mask, for example, to extend the etch depth to a level that the original photomask thickness cannot support. A feature can be etched by repetitively depositing the mask and etching the feature until the desired depth is achieved. The self-aligned mask can be used to clean the base (i.e., bottom of the partially etched feature), or extend the etch depth of an etched opening while protecting the exposed surface of the substrate and upper corner of the opening.
In one embodiment of the method of the invention, a self-aligning etch mask is used to continue the etch of an etched feature that is initially formed to a first selected depth in the substrate using a photoresist or other masking layer deposited over a substrate. The first depth of the etched feature is sufficient such that substantially no polymer material is deposited on the substrate at the bottom portion of the etched feature in a subsequent polymer deposition step. Preferably, the aspect ratio of the etched feature (i.e., ratio of the depth to the diameter of the feature) is at least about 0.5, preferably greater than about 0.5, more preferably greater than about 1.
According to the invention, the self-aligned, etch resistant mask layer is formed over the masking layer and the etch is continued. It is preferred that the self-aligned mask layer is formed over a portion of the sidewalls of the etched feature contiguous with the surface of the substrate to a limited depth into the etched feature, to cover and protect the comers and the sidewalls of the substrate from being eroded during a subsequent etch step.
Preferably, the self-aligned mask layer is deposited from a polymerizing gas by plasma enhanced chemical vapor deposition using a dual source high density plasma etcher, to form an etch resistant polymer material layer of a predetermined thickness. Exemplary polymerizing gases include fluorocarbons, hydrofluorocarbons, and chlorofluorocarbons, among others. A second etching can then be performed to extend the etched feature downwardly to a selected second depth.
During the deposition step, the thickness of the self-aligned mask can be controlled and modified to provide an effective covering to prevent etching of the substrate, while maintaining the mouth of the etched feature at a suitable width for a subsequent etch of the feature, by varying the bias power, the source power, and/or the deposition time configurations of the etcher. In addition, the etch selectivity/resistivity of the mask layer can be controlled by varying the setting for the source power of the etcher. The use of power to accomplish the self aligned deposition is preferred because power can be changed almost instantly and is, in most processing tools, well controlled and repeatable. It may be necessary to change other variables, most notably gas flow rate or gas pressure, to accomplish the self aligned deposition.
The self-aligned mask can thus be deposited on a layer of photoresist or other masking layer to continue the etch of a formed feature in a substrate (e.g., oxide, among others). In another embodiment, the self-aligned mask can also be formed over a masking layer that has been patterned with openings to expose an underlying substrate (e.g., oxide layer) that is to be initially etched to form a feature. Preferably, the openings of the mask layer have an aspect ratio at least about 0.5, preferably greater than about 1, such that polymer material deposited from a polymerizing gas is not deposited onto the substrate at the bottom of the mask opening when the self-aligned mask layer is formed.
In another embodiment of the method of the invention, a self-aligning etch mask is used to remove an etchable material from the bottom portion of an etched feature, while protecting the exposed surface of the substrate and upper corners of the feature. For example, a substrate having an etched feature and an overlying layer of an etchable material (e.g., silicon nitride, polysilicon) formed over the surface of the substrate including the sidewalls and bottom portion of the feature, can be processed to remove a portion of the etchable layer from the bottom of the etched feature. Preferably, the aspect ratio of the feature is at least about 0.5, preferably greater than about 0.5, more preferably greater than about 1, such that substantially no polymer material is deposited onto the substrate at the bottom portion of the feature in a subsequent polymer deposition step.
A self-aligned, etch resistant mask layer can then formed to a selected thickness over the upper surface of the etchable layer and, preferably contiguously over the comers of the etchable layer and extending into the etched feature for a limited depth. The mask layer effectively protects the etchable layer formed on the upper surface and along the sidewalls of the etched feature from being removed/damaged in a subsequent etch step. Preferably, substantially no polymeric material is deposited onto the etchable layer overlying the bottom portion of the opening. The self-aligned mask layer can be deposited from a polymerizing gas such as a fluorine-, chlorine-, or hydrocarbon-containing gas, and mixtures thereof, by plasma enhanced chemical vapor deposition using a dual source high density plasma etcher. After applying the self-aligned mask layer, the etchable layer overlying the bottom portion of the etched feature is then removed to expose the substrate on the bottom portion of the etched feature. The feature can also be extended past the etchable layer to result in a feature with sidewalls of differing materials. The thickness of the self-aligned mask can be modified by varying the bias power, the source power, and/or the deposition time configurations of the etcher. The etch selectivity/resistivity of the self-aligned mask layer can be modified by varying the configuration for the source power of the etcher.
Etching a deep feature utilizing the present invention, requires depositing only a thin photolithographic mask to start the etch and to set the stage for the sequential self-aligned mask deposition and etch sequences to increase the etch depth of the feature. The use of the self-aligned mask greatly simplifies the lithographic process by reducing the required thickness of the photoresist mask.
Another process capability enabled by the invention is the ability to clean the base or bottom portion of an etched feature, for example, a contact that has damaged silicon at the bottom of the opening, while protecting the exposed surface of the substrate and upper corner of the etched feature. This allows the cleaning step to be anisotropic and conserves the cd budget. It also allows the use of a cleaning agent that could otherwise remove surface material from the exposed surface of the structure.